1. Field of the Invention
The present invention relates to a method of estimating the fuel/air ratio of each cylinder of a fuel injected internal-combustion engine from a fuel/air ratio measurement downstream from the manifold and an extended Kalman filter.
2. Description of the Prior Art
Knowledge of the fuel/air ratio, characterized by the ratio of the mass of fuel to the mass of air, is important for all vehicles, whether equipped with gasoline engines, since it conditions good combustion of the mixture when it is close to 1, or with diesel engines, in which case the interest of knowing the fuel/air ratio is different insofar as diesel engines run under lean mixture conditions (ratio below 1). In particular, catalysts using a NOx trap lose efficiency in the course of time. In order to recover optimum efficiency, the fuel/air ratio has to be maintained close to 1 for some seconds, prior to returning to normal running conditions with a lean mixture. Depollution by DeNox catalysis therefore requires precise control of the fuel/air ratio cylinder by cylinder.
A probe arranged at the turbine outlet (supercharged engine) and upstream from the NOx trap therefore gives a measurement of the mean fuel/air ratio as a result of the exhaust process. This measurement being highly filtered and noise-affected is used for control of the masses injected into the cylinders during fuel/air ratio phases of 1, each cylinder receiving then the same mass of fuel.
In order to control more precisely, and in particular individually, injection of the fuel masses into the cylinders, reconstruction of the fuel/air ratio in each cylinder is necessary. Since installing fuel/air ratio probes at the outlet of each cylinder of a vehicle cannot be done considering their cost price, setting an estimator working from the measurements provided by a single probe advantageously allows to separately know the fuel/air ratios in each cylinder.
An engine control can thus, from the reconstructed fuel/air ratios, adjust the fuel masses injected into each cylinder so that the fuel/air ratios are balanced in all the cylinders.
French Patent 2,834,314 describes the definition of a model, based upon observation and filtering by means of a Kalman filter. This model contains no physical description of the mixture in the manifold and does not take into account highly pulsating flow rate phenomena.
Estimation of the fuel/air ratio in the cylinders is only conditioned by the coefficients of a matrix, coefficients that can be identified off-line by means of an optimization algorithm. Furthermore, a different adjustment of the matrix, therefore an identification of its parameters, corresponds to each working point (engine speed/load). This estimator thus requires heavy acquisition test means (with 5 fuel/air ratio probes) and has no robustness in case of engine change.